This invention relates to the continuous melting of a metallic charge to form a molten steel product. More particularly, it relates to the continuous preheating of charge materials for continuous steelmaking. Continuous steelmaking is particularly advantageous in those regions where there is a concentration of production of, or ready availability of scrap and/or direct reduced iron (DRI), and where electric energy is both available and economical.
Heretofore, the operation of an electric arc steelmaking furnace has been an intermittent operation, wherein the sequence followed is: charging of steel scrap and/or direct reduced iron, pig iron, slag formers and alloying elements; ignition or establishment of an electric arc between the electrodes in the furnace to create melting conditions for melting the charge and forming a molten metal bath covered by a molten slag; refining for a period of time during which the molten metal portion of the bath is refined to form steel having a desired composition and quality; and periodically raising the electrodes to remove them from contact with the bath and interference with the tapping procedure; and then tapping the molten metal. In addition, slag can be removed by a slagging, or slag-off, operation as required.
Electric steelmaking technology has been undergoing radical changes for the past twenty years. The success of ladle refining for normal steel quality requirements and secondary refining for high quality requirements have increased furnace productivity, and are influencing furnace design and operation.
Between fifteen and twenty years ago, the time consuming double slag practice was replaced by rapid metallurgy, resulting in some operations having power on during up to 70 percent of the tap-to-tap time, and 70 percent of the power-on time operating at full transformer capacity.
A short time later, a productivity of one metric ton of cast steel per MVA/hour was achieved by utilizing the ultra-high power concept. However, this productivity level is still a goal for most electric steelmakers. More recently, a few steelmakers have attained a productivity of about 1.8 metric tons of cast steel per MVA/hour by combining ultra-high power with scrap preheating, oxygen lancing, oxy-fuel burners, and ladle metallurgy. Tap to tap time ranges from about 60 to 80 minutes and a somewhat unstable equilibrium is reached with both the cycles of the furnace and the caster. Even today, there is still an unstable equilibrium, because it is reached under optimum conditions of furnace operation with only minimal allowance for the many unpredictables of batch furnace operation. Thus, long sequential casting from the EAF into a continuous caster is not a common practice, but the exception.
I have invented a better method of operating an electric arc steelmaking furnace, which incorporates continuous preheating, feeding and melting, and results in an increase in quality and productivity, and reduced operating costs. This method results also in a truly continuous operation of the caster, thus insuring a continuous output of cast steel during the whole refractory campaign of the furnace. Therefore, the invention can be characterized as a method for continuous steelmaking.
Although the present invention is shown and described in connection with an electric arc steelmaking furnace, it will be readily apparent that any electric powered steelmaking furnace including but without limitation, plasma furnaces and induction furnaces could be substituted for the electric arc steelmaking furnace with like results.
There is currently a steelmaking practice known as "continuous charging" or "continuous melting", but these practices refer to a charging practice in which charge materials are fed to a furnace during the charging, melting and refining periods, then charging is interrupted and power input is interrupted for the tapping procedure. It has been found that an electric steelmaking furnace can be operated continuously without interruption of charging or power input for the tapping procedure by taking the following steps in the steelmaking process.
First, if the furnace is of a small size, scrap must be prepared by shredding or shearing it to a suitable size. The scrap is preferably segregated for quality control. Segregation of scrap eliminates or limits undesirable elements, and classifies and makes available valuable alloy constituents. For example, copper is a strong contaminant in deep drawing steels, but is a desirable addition for weathering steels such as COR-TEN steel (see Making, Shaping and Treating of Steel, pages 572-73, 9th edition, 1971). As received, the scrap is segregated into desired classifications, preferably depending on contamination by tramp elements sulfur and phosphorus. Segregated scrap is shredded or sheared and stored for use. By maintaining a stock of shredded or sheared raw material, continuous operation of the process is assured during periods of shredder or shear down-time.
Although prepared scrap is mandatory for small furnaces, commercial scrap can be fed to medium and large furnaces without preparation. The requirement for shredded or sheared scrap is strictly related to the furnace size. Furnaces of 3 meter diameter or smaller (small furnaces) require scrap of a maximum longest dimension of about one foot (0.3 meter). Furnaces of 5 meter diameter or larger (large furnaces) can be fed commercial scrap such as heavy melting Number 1 or No. 2, plate and structural scrap, and any equivalent sized scrap. Medium sized furnaces, between 3 to 5 meter diameter, should be fed a mix of shredded, sheared, and commercial scrap.
Direct reduced iron is normally prepared in the form of lumps or pellets, which are generally of a size of less than about one half inch diameter. Direct reduced iron briquets can also be used as feed material. Preferably such direct reduced iron is produced at a contiguous plant.
Scrap, direct reduced iron, slag formers and alloying materials are preheated in accordance with this invention, and continuously fed to the electric arc furnace. A foaming slag practice is used, and the furnace is only partially tapped intermittently without removal of the electrodes, thus electrodes remain at full power during both continuous feeding, refining (which is continuous) and tapping (which is intermittent). Tapping is carried out by limited tilting of the furnace, generally not varying more than 15.degree. from the vertical.